1. Field of the Invention
This invention is related to apparatuses and methods for forming a window in a casing in a wellbore.
2. Description of Related Art
The practice of producing oil from multiple radially dispersed reservoirs, through a single primary wellbore has increased dramatically in recent years. To facilitate this, "kick-off" technology has been developed and continues to grow. This technology allows an operator to drill a vertical well and then continue drilling one or more angled or horizontal holes off of that well at chosen depth(s). Because the initial vertical wellbore is often cased with a string of tubular casing, a "window" must be cut in the casing before drilling the "kick-off". In certain prior art methods windows are cut using various types of milling devices and one or more "trips" of the drill string are needed. Rig. time is very expensive and multiple trips take time and add to the risk that problems will occur.
Another problem encountered in certain typical milling operations is "coring". Coring occurs when the center line of a window mill coincides with the wall of the casing being milled (i.e. the mill is half in and half out of the casing). As the mill is rotating, the point at its centerline has a velocity of zero. A mill's capacity to cut casing depends on some relative velocity between the mill face and the casing being cut. When the centerline of the mill contacts the casing wall its cutting capacity at that point is greatly reduced because the velocity near the centerline is very low relative to the casing and zero at the axial centerline. The milling rate may be correspondingly reduced.
Milling tools are used to cut out windows or pockets from a tubular, e.g. for directional drilling and sidetracking; and to remove materials downhole in a well bore, such as pipe, casing, casing liners, tubing, or jammed tools. The prior art discloses various types of milling or cutting tools provided for cutting or milling existing pipe or casing previously installed in a well. These tools have cutting blades or surfaces and are lowered into the well or casing and then rotated in a cutting operation. With certain tools, a suitable drilling fluid is pumped down a central bore of a tool for discharge adjacent or beneath the cutting blades. An upward flow of the discharged fluid in the annulus outside the tool removes cuttings or chips from the well resulting from the milling operation.
Milling tools have been used for removing a section of existing casing from a well bore to permit a sidetracking operation in directional drilling and to provide a perforated production zone at a desired level. Also, milling tools are used for milling or reaming collapsed casing and for removing burrs or other imperfections from windows in the casing system.
Prior art sidetracking methods use cutting tools of the type having cutting blades. A deflector such as a whipstock causes the tool to be moved laterally while it is being moved downwardly in the well during rotation of the tool to cut an elongated opening pocket, or window in the well casing.
Certain prior art well sidetracking operations which employ a whipstock also employ a variety of different milling tools used in a certain sequence. This sequence of operation may require a plurality of "trips" into the wellbore. For example, in certain multi-trip operations, an anchor, slip mechanism, or an anchor-packer is set in a wellbore at a desired location. This device acts as an anchor against which tools above it may be urged to activate different tool functions. The device typically has a key or other orientation indicating member. The device's orientation is checked by running a tool such as a gyroscope indicator or measuring-while-drilling device into the wellbore. A whipstock-mill combination tool is then run into the wellbore by first properly orienting a stinger at the bottom of the tool with respect to a concave face of the tool's whipstock. Splined connections between a stinger and the tool body facilitate correct stinger orientation. A starting mill is releasably secured at the top of the whipstock, e.g. with a shearable setting stud and nut connected to a pilot lug on the whipstock. The tool is then lowered into the wellbore so that the anchor device or packer engages the stinger and the tool is oriented. Slips extend from the stinger and engage the side of the wellbore to prevent movement of the tool in the wellbore; and locking apparatus locks the stinger in a packer when a packer is used. Pulling on the tool then shears the setting stud, freeing the starting mill from the tool. Certain whipstocks are also thereby freed so that an upper concave portion thereof pivots and moves to rest against a tubular or an interior surface of a wellbore. Rotation of the string with the starting mill rotates the mill. The starting mill has a tapered portion which is slowly lowered to contact a pilot lug on the concave face of the whipstock. This forces the starting mill into the casing and the casing is milled as the pilot lug is milled off. The starting mill moves downwardly while contacting the pilot lug or the concave portion and cuts an initial window in the casing. The starting mill is then removed from the wellbore. A window mill, e.g. on a flexible joint of drill pipe, is lowered into the wellbore and rotated to mill down from the initial window formed by the starting mill. A watermelon mill may be used behind the window mill for rigidity; and to lengthen the casing window if desired. Typically then a window mill with a watermelon mill mills all the way down the concave face of the whipstock forming a desired cut-out window in the casing. Then, the window mill is removed and, as a final option, a new window mill and string mill and a watermelon mill are run into the wellbore with a drill collar (for rigidity) on top of the watermelon mill to lengthen and straighten out the window and smooth out the window-casing-open-hole transition area. The tool is then removed from the wellbore.
The prior art discloses a variety of chemical and explosive casing cutters and casing perforators. These apparatuses are used to sever casing at a certain location in a wellbore or to provide perforations in casing through which fluid may flow.
There has long been a need for efficient and effective wellbore casing window methods and tools useful in such methods particularly for drilling side or lateral wellbores. There has long been a need for an effective "single trip" method for forming a window in wellbore casing.